In Mark 4: 1-20, after Jesus tells the parable of the sower, he then explains the meaning of some of the imagery in the parable but not every one of them. He explains the meaning of the birds devouring the seed (Satan takes away the Word sown in some people). He explains the meaning of the plants that wither (some people fall away from the Word because of trouble or persecution). He explains the meaning of the plants choked by thorns (worries, wealth, or materialism crowd out the Word and make those people unfruitful.)
Jesus explains that the plants yielding fruit, springing up, and increasing, are the people who hear the Word, receive it, and fruiten (καρποφοροῦσιν). Matthew’s version of the parable is similar -- the good-soil people hear the Word, synthesize it, and fruiten, while in Luke the good-soil people hear the Word, retain it, and fruiten.
Jesus does not explain the meaning of the 30-fold, 60-fold, and 100-fold differences between the good-soil people. What accounts for these different levels of productivity? The growing conditions are the same. Why do some yield twice as much and some more than three times as much fruit as others?
Furthermore, what exactly is being produced? I think the fruits referred to a life that is increasingly lived according to the example set by Christ. I checked some commentaries for alternate interpretations to see if anyone thought it was an evangelism reference (i.e., some would convert 30-fold, etc). The consensus among the sources I checked (including Wesley) was consistent with my initial impression.
A καρποφοροῦσιν (fruit-bearing) person is one whose life demonstrates evidence of Christian virtue. The fruity evidence will display itself in many places -- the personality, the disposition, the deeds, the activities, the behaviors, the values (just to name a few).
At first, I felt sadness over the fact that some people only produced a 30-fold or 60-fold virtuous life. What kept them from continuing to grow until they reached 100-fold? Then I noticed that Jesus didn’t seem to mind the varying levels of production. He doesn’t criticize the 30-fold and 60-fold people. While (in comparison to the 100-fold producers) there’s not as much evidence that the gospel has taken root in their lives, Jesus still considers them examples of good-soil people.
Quantity doesn’t matter in this parable. Καρποφοροῦσιν is the point. People whose lives evidence some Christian virtues rather than none, that’s the happy ending to the parable.